The internal combustion engine industry is under ever increasing pressure to reduce pollution to the environment by lowering harmful engine emissions. One response to this pressure has resulted in research into adapting compression ignition (CI) engines (also known as “diesel” engines) to burn natural gas instead of diesel fuel. Compared to diesel fuel, natural gas is a relatively clean burning fuel and the substitution of natural gas for diesel fuel can reduce emission levels of both nitrogen oxides (NOx) and particulate matter (PM).
A known technique for substituting natural gas for diesel fuel is called dual fuel operation. In this method, natural gas is mixed with intake air prior to introducing the air/natural gas mixture into the engine cylinder (a process known in the art as fumigation). The mixture is then introduced into the piston cylinder during the intake stroke. During the compression stroke, the pressure and temperature of the mixture are increased. Near the end of the compression stroke, dual fuel engines inject a small quantity of pilot diesel fuel to ignite the mixture of air and natural gas. Combustion is triggered by the auto-ignition of the diesel fuel and it is believed that a propagation combustion mode occurs under these conditions. One advantage of employing a pre-mixed charge of air and natural gas is that the fuel to air ratio can be lean. With fumigation it is possible to realize the advantages of “lean burn” operation, which include lower NOx emissions, lower PM and a potentially higher cycle efficiency.
Known dual fuel methods, however, have at least two disadvantages. One disadvantage is encountered at high load engine operating conditions, when the elevated temperature and pressure in the piston cylinder during the compression stroke makes the air/natural gas mixture susceptible to “knocking”. Knocking is the uncontrolled auto-ignition of a premixed fuel/air charge. Knocking leads to a rapid rate of fuel energy release that can damage engines. Measures to reduce the risk of knocking include lowering the compression ratio of the piston stroke or limiting the power and torque output. These measures, however, cause a corresponding reduction in the engine's cycle efficiency (that is, not as much power is available from each piston stroke).
A second disadvantage of known dual fuel methods is that under low load engine operating conditions, the mixture of fuel and air becomes too lean to support stable premixed combustion and results in incomplete combustion or misfiring. The intake air flow can be throttled to maintain a mixture that can sustain premixed combustion, but throttling adversely affects engine efficiency.
A second type of engine substitutes gaseous fuel for diesel fuel in an internal combustion engine is sometimes referred to as a “high pressure direct injection” engine. Similar to conventional dual fuel engines, which employ the above-described method, a large fraction of the fuel burned in high pressure direct injection engines is gaseous, yielding an improvement (over engines burning only diesel fuel) with respect to the emission levels of NOx and PM. In addition, high pressure direct injection engines have the potential to achieve the same cycle efficiency, power and torque output as counterpart conventional diesel-fuelled engines. The operational principle underlying high pressure direct injection engines is that two fuels are injected under pressure into the engine cylinder near the end of the compression stroke. According to one method, a small quantity of “pilot fuel” (typically diesel) is injected into the cylinder immediately followed by a more substantial quantity of gaseous fuel. The pilot fuel readily ignites at the pressure and temperature within the cylinder at the end of the compression stroke, and the combustion of the pilot fuel initiates the combustion of the gaseous fuel. Accordingly, high pressure direct injection engines generally have little or no pre-mixture of gaseous fuel and air, and thus the gaseous fuel burns in a “diffusion” combustion mode, rather than a premixed combustion mode.
A two-fuel injector is used to provide a means to practise high pressure direct injection combustion. The two-fuel injector can independently inject a pilot and a gaseous fuel.
An advantage of high pressure direct injection engines over conventional dual fuel mode operation is that they are not susceptible to knocking under high load conditions because the air and gaseous fuel are not pre-mixed and the gaseous fuel is not introduced into the cylinder until after the pilot fuel. Another advantage of high pressure direct injection engines is the ability to operate under low load conditions without the need to throttle the engine.
In addition to the dual fuel and high pressure direct injection combustion modes, the HCCI combustion mode is available for gaseous fuels in internal combustion engines. Homogeneous charge compression ignition (HCCI) is an alternative to the propagation mode of combustion for providing a mode of lean burn pre-mixed combustion. Experimental HCCI engines generally introduce a homogeneous mixture of fuel and air into the engine cylinder(s). Under certain conditions, compression heating of the charge leads to ignition throughout the bulk of the pre-mixed charge without flame propagation, and this combustion mode is defined herein as HCCI. HCCI is essentially a “controlled knock” condition where the combustion rate is mainly controlled by the chemical reaction kinetics. HCCI is thus distinct from a combustion mode controlled by flame propagation. In a flame propagation combustion mode, when a homogeneous mixture of fuel and air is sufficiently rich to sustain a flame and is ignited at a point, a flame front forms and advances from the ignition point. In a flame propagation combustion mode, the rate of combustion is limited by the transfer of the unburned mixture of fuel and air into the flame reaction zone.
An advantage of a HCCI combustion mode is that very lean mixtures of fuel and air mixtures can be burned. For example, a fuel/air equivalence ratio of between 0.1 to 0.5 can burn in a HCCI combustion mode, whereas under the same conditions, in a propagation combustion mode combustion would be unstable, leading to misfire or partial burn. With a HCCI combustion mode, under very lean conditions, NOx formation rates can be substantially reduced relative to more typical lean burn flame propagation combustion modes.
With a HCCI engine the rate of combustion is potentially very rapid, resulting in high engine cycle efficiencies (relative to a conventional diesel-fuelled engine). However, a disadvantage of HCCI combustion is the lack of direct control over the start and rate of combustion because only indirect control methods are available. Recent studies of HCCI combustion show that the main control strategies over HCCI mode combustion include:    (a) using variable intake manifold temperatures (cooled exhaust gas recirculation (EGR) and intake air heating);    (b) using residual gas trapping;    (c) controlling intake manifold pressure;    (d) controlling premixed charge fuel/air equivalence ratio;    (e) controlling fuel type and blend;    (f) using a variable compression ratio;    (g) using EGR rates to control rate of combustion;    (h) addition of water in the intake charge to control the rate of combustion.
Another disadvantage of HCCI combustion is that at high load conditions, the higher fuel/air ratios result in HCCI combustion rates which can cause engine damage by combusting too rapidly, or by the rate of combustion causing very high in-cylinder pressures. Extending the operable range for HCCI combustion has been achieved through supercharging; use of EGR to reduce rate of heat release; late injection of diesel fuel; varying the compression ratio; and injection of gaseous fuel near top dead centre of the compression stroke. The result is two separate combustion modes in the same engine cycle. The two combustion mode approach, referred to here as HCCI-DI combustion, allows the operable power range of the engine to be extended while still benefitting from the low NOx and high efficiency of the HCCI event.
As discussed above, HCCI events on their own or in combination with a diffusion combustion mode, require a means to control the start of combustion. With the advent of the two fuel injector used for fuelling high pressure direct injection engines, one means available to control the start of the HCCI event is through the use of a pilot fuel. Here, consideration is given to a means of using a pilot fuel to control the HCCI event in the HCCI or HCCI-DI engine.